Various methods and apparatus have been proposed for separating plastic and/or plastic/metal composites from mixed waste materials which contain one or more sources of plastic and/or plastic/metal composites, such as milk containers, gable-top cartons, aseptic packages, and the like, in order to obtain substantially pure plastic which can be readily recycled to high grade end uses which call for plastic having, e.g., the ability to be blow-molded or the ability to be extruded into thin films, as opposed to low grade end uses such as plastic lumber, and/or in order to obtain plastic/metal composites which have a sufficiently high metal content to be suitable for use in the economical recovery of the pure metal. Preferably, the substantially pure plastic can be recycled into high grade end uses such as laminated paperboard products.
The source of plastic and/or plastic/metal composite may also contain, and often does contain, an amount of paper fiber. Methods and apparatus have been proposed for recovering this paper fiber in substantially pure form in order to provide additional recyclable paper fiber which can be readily recycled, also preferably to high grade end uses such as laminated paperboard products.
For example, Brooks, U.S. Pat. No. 3,741,863, discloses a method of separating and recycling cellulose fibers from waste by heating and abrading the cellulose material and then softening any resins contained therein. The separated fibers are then combined with a resin to form a mat which is compressed to form a board.
Laundrie, U.S. Pat. No. 3,814,240, discloses a method of separating waste paper from a thermoplastic film using a hot gas stream to contract the plastic and make it easier to mechanically remove the plastic particles.
Marsh, U.S. Pat. No. 3,925,150, describes a method of separating waste corrugated paperboard into neutral sulfite semi-chemical pulp and short fiber constituents. The waste is processed through two pulpers with a liquid cyclone and screening stage in between.
Tra, U.S. Pat. No. 4,017,033, addresses the problem of separating heavy and light contaminants from partially liberated fibrous materials. A centrifugal separator is used to create a high speed vortex which allows the heavy contaminants to be removed as a bottoms. A further separation means is used to remove the light contaminants.
Ortner, et al., U.S. Pat. No. 4,231,526, disclose the treatment of waste paper to separate both light and heavy foreign matter. The light foreign matter is sent to a hydrocyclone from which the recovered fibers are sent to a storage facility.
Espenmiller, U.S. Pat. No. 4,272,315, discloses a process of recovering fiber from waste paper by continuously removing plastic and lightweight trash. The pulper is equipped with different sized extraction holes to facilitate separation.
Heinbockel, et al., U.S. Pat. No. 4,283,275, disclose an auxiliary circulation system comprising a stock pulper equipped with a rotor and two screens having different sizes of mesh for the removal of contaminants from a stock suspension. Means are included to channel the stock suspension to a papermaking machine and to provide an auxiliary circuit for the removal of contaminants and rejects such as plastics and foils.
Cerroni, U.S. Pat. No. 4,314,674, discloses a method of separating an urban waste mixture of paper and plastic. The waste is initially separated and then a mix of paper and plastic film is ground such that the size of the paper is reduced without affecting the size of the plastic.
Zentgraf, et al., U.S. Pat. No. 4,570,861, disclose a method of separating paper and plastic by comminuting the mixture, triboelectrically charging the comminuted mixture and then letting the mixture pass through a free-fall separator to thereby separate the two components. An optional second electrostatic separation step may be employed.
The foregoing methods do not address the significant problems that arise when attempting to process mixed waste materials in order to obtain paper fiber and/or plastic suitable for recycling to high grade end uses, such as in laminated paperboard products in which the paperboard must meet certain brightness and dirt content requirements, and in which a plastic film is laminated to paperboard. These problems are further complicated when attempting to process mixed waste materials containing metal foil to obtain paper fiber and/or plastic suitable for use in laminated products which also include a layer of metal foil.
Laminated paperboard products are found in many forms and represent a high grade end use for recycled paper fiber and plastic due to the relatively stringent quality requirements of the several components of the products.
Laminated paperboard products are used as a packaging material, particularly in the form of cartons for storing consumable liquids like juices and milk. Cartons which generally have reclosable upper ends are typically laminated products made of paper fiber laminated on both sides with a layer of plastic such as polyethylene. This type of carton, known as a gable-top carton, is generally stored and sold refrigerated, and is typically refrigerated during use. A typical construction is a three-layer laminate having the sequential layers: polyethylene/fiber/polyethylene.
Aseptic cartons are those which are intended to have a longer shelf life. One type of aseptic carton is filled under aseptic conditions, so that the liquid contents completely fill the carton and there is no air or gas above the liquid. Filling the cartons under these aseptic conditions provides a long shelf life for the contents. The packaging may include a barrier layer which prevents oxidation of the liquid contents. This type of carton is typically provided with an area that may be punctured for inserting a straw. The straw remains in the puncture hole until the contents are consumed. The most common commercial example of an aseptic carton is an individual juice container, commonly known as a "juice box". Aseptic cartons are laminated products which include both paper fiber and plastic layers as well as a thin metal foil layer, such as aluminum foil. A typical construction includes layers of polyethylene separately sandwiching layers of paper fiber and aluminum foil, to form a five-layer laminate having the sequential layers: polyethylene/paperboard/polyethylene/foil/polyethylene.
Laminated paperboard that is used to make gable-top cartons and aseptic packages must satisfy stringent requirements. These requirements have heretofore limited, or excluded, recycled paper fiber and/or recycled plastic from being used to make such laminates.
The paper fiber used in these laminated paperboard products generally is required to be of high brightness and low in dirt content. Elevated dirt count can cause streaking problems during clay coating of the board and also may not be acceptable from an appearance point of view. Further, the paper fiber used in these laminated paperboard products must have sufficient brightness so as not to have adverse impact on the brightness of the resulting paperboard; otherwise, the resulting paperboard may not be amenable to accepting printing and/or may not be aesthetically pleasing. The board must also meet certain levels of physical strength properties, like stiffness, etc.
The plastic used in these laminated paperboard products generally is required to be very low in contaminants as well. One of the properties required of the plastics used in these laminated paperboard products is the ability to be extruded into thin films. Contaminants in the plastic, such as residual paper fiber and/or residual metal and/or elevated moisture content (which may be due in part to moisture absorption by residual paper fibers) disrupt the film-forming ability of the plastic.
With the continuing growth of the laminated paperboard product industry, particularly for packaging of consumer goods, there is an increasing need for a process of processing mixed waste materials to obtain recycled paper fiber and/or recycled plastic which can be utilized in laminated paperboard products.
Also, because laminated paperboard products such as gable-top and aseptic cartons contain plastic and/or plastic/metal composites, they are not biodegradable. With increasing concern over the environment, there is also growing pressure to find a way to separate and recycle the components of laminated paperboard products, particularly the paper fiber and plastic portions. Preferably, a method for processing mixed waste materials should allow the recycled paper fiber and/or recycled plastic to be reused in the same or similar product from which it was derived, i.e., the so-called "carton-to-carton" recycling.